The present invention relates to a method of producing a spheroidal graphite cast iron article excellent in bending characteristics and mechanical strength such as impact strength.
Since spheroidal graphite cast iron has an excellent mechanical strength and castability, it is widely used in various applications including automobile parts, machine parts, etc. Specifically, spheroidal graphite cast iron species of FCD 700 and FCD 800 in JIS G5502 are used for parts requiring a high mechanical strength, and spheroidal graphite cast iron species of FCD 370 and FCD 400 in JIS G5502 are used for parts requiring a large elongation. Further, since important parts of automobiles such as suspension parts are required to have good properties in tensile strength, elongation, fatigue resistance, impact strength, etc., the spheroidal graphite cast iron constituting such important parts should satisfy the above strength requirements. However, the as-cast surface of the spheroidal graphite cast iron has some unevenness due to sand inclusion and slag inclusion, and such an unevenness is likely to function as starting points of cracking and failure. Therefore, the spheroidal graphite cast iron having an as-cast surface fails to exhibit its inherent mechanical strength sufficiently.
In such circumstances, one of the inventors has previously proposed in U.S. Pat. No. 4,990,194 a thin spheroidal graphite cast iron article having a good mechanical strength, which has graphite particles dispersed in a ferrite matrix containing 10% or less of pearlite by volume fraction, and is characterized in that there is substantially no fine gap between the graphite particles and the ferrite matrix. Such a thin high-strength article of spheroidal graphite cast iron can be produced by pouring a melt having a spheroidal graphite cast iron composition into a casting mold; removing the casting mold by shake-out after the completion of solidification of the melt, while substantially the entire portion of the resulting cast iron product is still at a temperature of its A.sub.3 transformation point or higher; introducing the cast iron product into a soaking zone of a continuous furnace kept at a temperature of the A.sub.3 transformation point or higher, where the cast iron product is kept for 30 minutes or less to decompose cementite contained in the matrix; and transferring the cast iron product into a cooling zone of the continuous furnace to cool the cast iron product at such a cooling speed as to conduct the ferritization of the matrix.
However, unlike in the case of the thin articles of spheroidal graphite cast iron, spheroidal graphite cast iron articles having relatively large thickness, at least about 10 mm thickness for example, for use in parts which should satisfy higher mechanical strength requirements should retain a pearlite phase to show a good mechanical strength and at the same time should exhibit improved bending characteristics. For this purpose, the heat treatment of ferritizing the spheroidal graphite cast iron entirely or mostly is not satisfactory.
To solve this problem, one of the inventors has previously proposed in U.S. Pat. No. 5,346,561 a spheroidal graphite cast iron article which has a surface layer portion mostly composed of a ferrite phase and having a thickness of at least 1 mm, and an inner portion composed of a pearlite phase and a ferrite phase, the surface layer portion having a ferritization ratio of 70% or more which is larger than that of the inner portion by at least about 15%.
Such a spheroidal graphite cast iron article can be produced by the steps of (a) pouring a melt having the same composition as that of a spheroidal graphite cast iron to be produced into a casting mold; (b) removing the casting mold by shake-out after the completion of solidification of the melt, while substantially the entire portion of the resulting cast iron product is still at a temperature of its Al transformation point or higher; (c) when the temperature difference between the surface layer portion and the inner portion has become 40.degree. to 60.degree. C., introducing the cast iron product into a soaking furnace kept at a temperature of 750.degree. to 900.degree. C., where the cast iron product is held for such a period of time as to produce the surface layer portion having a ferritization ratio of 70% or more which is larger than that of the inner portion by at least about 15%; and (d) transferring the cast iron product into a cooling furnace to cool the cast iron product at a cooling speed of 15.degree. to 100.degree. C./min.
Another method of producing such a spheroidal graphite cast iron article comprises the steps of (a) introducing a pearlitized spheroidal graphite cast iron product into a soaking furnace kept at a temperature of 780.degree. to 870.degree. C., where the cast iron product is held for such a period of time as to produce the surface layer portion having a ferritization ratio of 70% or more which is larger than that of the inner portion by at least about 15%; and (b) transferring the cast iron product into a cooling furnace to cool the cast iron product at a cooling speed of 15.degree. to 100.degree. C./min.
Although the spheroidal graphite cast iron articles produced by the above methods have good bending characteristics and a high mechanical strength, both the methods involve the steps difficult to be strictly controlled. More specifically, in the first method, a cast iron product after the shake-out is introduced into the soaking furnace kept at 750.degree. to 900.degree. C. when the temperature difference between the surface layer portion and the inner portion reaches 40.degree. to 60.degree. C. However, it is very difficult to determine the timing for introducing the cast iron product to the soaking furnace. In the second method where a pearlitized spheroidal graphite cast iron product is subjected to ferritization in the soaking furnace kept at 780.degree. to 870.degree. C., the holding time in the soaking furnace for reaching a intended ferritization ratio is difficult to be suitably controlled.